Typically, the secondary battery is capable of being recharged and being large-scaled. Cadmium nickel, hydrogen nickel and lithium ion battery can be taken as representative examples. Amongst them, the lithium ion battery is promising as next-generation power source because it has superior characteristics such as longevity and high capacity. However, if the lithium ion battery is exposed to abnormal usage environment such as overcharge, short-circuit, reverse-connection and heat-exposure, the gas is generated within the battery due to electrochemical reaction, thereby increasing an internal pressure of the battery. The battery is swollen due to the increased internal pressure and particularly an electrolyte or an active material is partially decomposed to cause the internal pressure and temperature of the battery to be increased rapidly if the abnormal usage time such as overcharge is persisted, which results in danger of causing explosion and fire.
In order to verify the safety of the secondary battery, tests of overcharge, over-discharge, short-circuit and reverse-connection, as well as various heat stability tests of high temperature storage test, thermal shock test, and thermal exposure test are performed. The explosion or fire of the battery must not be included in conditions of such thermal stability tests.
An attempt to improve the stability of the secondary battery has been very widely made, and a method of exhausting gas generated within the secondary battery through a destruction unit of the battery case or a method of directly interrupting the battery circuit using a destruction disc within the battery has been developed. In this case, if the gas is generated in the condition such as overcharge to cause the internal pressure to exceed the design value, the spark generated at the time of destruction can serve as a source of ignition which causes explosion and a fire even though releasing the internal pressure and ensuring the stability in such a way that a sealing unit is destroyed or the power source of the battery is interrupted.
FIG. 1(a) is a cross-sectional view of prior secondary battery stability apparatus. As shown in FIG. 1(a), a secondary battery 4 configured of a case 2 and an electrode assembly 3 is housed in the secondary battery pack 1. A needle-type projecting portion 5 is equipped on the inside of the secondary battery pack 1. If the condition such as overcharge, short-circuit and reverse voltage occurs, a temperature of the secondary battery 4 increases and thus the electrolyte or the active material within the secondary cell 4 is converted into the gas phase, which results that the secondary cell 4 is swollen up.
If the secondary battery 4 is swollen up above the predetermined value, the explosion and fire of the secondary cell can be prevented by breaking the seal of the secondary battery using the needle-type projecting unit 5.
Since the prior technology of protecting the secondary battery 4 using the projecting portion 5 requires an additional production process, there are problems of decreasing productivity and not ensuring destruction reliability.
Further, there is a problem of contrary inconsistency that the stack portion must have sealing property and destruction property simultaneously.
There is a further problem in that a harmful gas is exhausted due to the explosion of the sealing unit caused by the internal pressure of the battery, which damages the electronic circuit and adversely affect the human body.
Since the case of a pouch-type secondary battery is formed of a flexible thin plate produced by mixing the metal material such as aluminum and resin material such as polymer resin, it is difficult to structure the safety apparatus according to the prior art.
FIG. 1(b) is a cross-sectional view of a safety apparatus of prior art rupture disc-type secondary battery. As shown in FIG. 1(b), a gas exhaust hole 7 is provided in a top portion of a cylindrical secondary battery and a cap cover 6 is separated from a cap 8 by a rupture disc 9. If an internal pressure of the cylindrical secondary battery increases, the internal pressure is delivered to the rupture disc 9 via the gas exhaust hole 7. If the internal pressure above the predetermined value is delivered, the rupture disc 9 is destroyed to cause the gas to be discharged so that a power source of the cylindrical secondary battery is interrupted, thereby preventing explosion and fire of the secondary battery.
The prior art using the rupture disc 9 has problems in that harmful gas is discharged when the disc is destroyed and operated and also the spark generated when the rupture disc 9 is destroyed serves as a fire source of the discharge gas, which results in fire and explosion. Further, there is a technological limit to directly interrupt the battery circuit in a case of the secondary battery for use in vehicle in which high voltage and large amount of current are applied.